I would envision something like this -minimum requisites to start training a trait. For example to start training for a marksman you need minimum 85 marksmanship. To start training as a machinegunner you need 80 marksmanship and 80 strength. And so on, and so on. You could retrain yourself, while keeping the trait limit. So going from marksman to heavy weapons, but not to bombardier, unless you lose another trait.